Detoxification of Plant Secondary Metabolites

At every meal, herbivores are faced with the prospect of being poisoned by naturally occurring toxins in plants. Little experimental work exists on the physiological mechanisms that mammalian herbivores employ to deal with plant toxins or how some species such as the koala or Stephen’s woodrat are capable of specializing on toxic plants.

My research focuses on ecological factors and physiological constraints that influence foraging behavior and the evolution of diet breadth in mammalian herbivores. My laboratory is currently investigating the evolution of dietary specialization in herbivores by exploring the detoxification abilities of specialist and generalist woodrats (Neotoma species). Woodrats are a model system because the diversity of specialists and generalists is unparalleled by any other genus of mammalian herbivores. Furthermore, because woodrats are closely related to laboratory rats, many of the molecular reagents designed for lab rats work on woodrats.

Using the woodrat system, my lab has found that specialist woodrats are capable of tolerating higher doses of toxins than generalist woodrats. Contrary to conventional wisdom, specialists do not appear to have unique detoxification pathways compared to generalists but rather specialists have detoxification systems with greater capacities. We are expanding on this work by taking a functional genomics approach. Detoxification is an energetically demanding process. Specialist herbivores are predicted to have evolved detoxification systems more efficient at processing the toxins from their natural diet than generalist herbivores processing the same compounds. How expensive is detoxification and are specialists more efficient?

Our results reveal that both specialist and generalist woodrats experience significantly greater energetic costs when consuming the toxic diet of the specialist compared with nontoxic, control diets. However, specialists expend far less energy per gram of toxin ingested than generalists. Specialists achieve energetic efficiency by decreasing metabolic rate and using less expensive detoxification pathways. Evolutionary theory predicts that specialist herbivores have evolved detoxification systems with a high capacity for the limited spectrum of toxins in their diet and thus should not be as capable of processing novel toxins as generalist herbivores. Our recent work supports this hypothesis. On a diet containing a novel toxin, the specialist woodrat did not perform as well as the generalist woodrat. We are expanding this work to include more specialist-generalist pairs and more novel toxins. Do herbivores regulate intake of plant toxins? Our work has shown that the desert woodrat, Neotoma lepida regulates the intake of creosote resin by reducing meal size as toxin concentration increases. We are currently investigating how woodrats make such foraging decisions.